Isotope values from milk and blood serum in New Zealand sea lions: are pups feeding on milk a trophic level higher than their mothers?

Abstract

Stable isotope analysis is increasingly used to investigate diet and foraging behaviour in marine wildlife. For marine mammals, samples from offspring are being used as proxies for their mothers foraging isotope niche as the transfer of nutrients between mother and neonates is solely through milk. Most of these proxy studies have, however, used only easily collected tissues, such as fur or whiskers. Quantifying the changes in δ13C and δ15N values between mother and pup requires the measurement of the mother/offspring pairs’ milk and blood serum. Here, δ13C and δ15N values were measured from serum and milk samples from lactating New Zealand (NZ) sea lion, Phocarctos hookeri females and their pups. The change in δ13C values between mother and pup serum averaged ~ 0.8‰. Whole milk was strongly δ13C depleted compared to serum δ13C values from either mothers or pups. The change in δ15N values averaged 1.3‰ between mother and pup serum, while differences in δ15N values from whole milk to mother or pup serum averaged 0.4‰ and 0.9‰, respectively. Lactating NZ sea lions are known to have two distinct foraging ecotypes which can be identified in their isotopic values and these differences could also be determined in the δ15N values of milk and serum samples from their pups. Similar to other research, the findings confirm that changes in δ13C and δ15N between tissues of mothers and pups are species and tissue specific, and greater research is needed to understand their physiological and biochemical isotopic relationship.